Since their commercialization by Sony Corporation in 1991, lithium ion secondary batteries have been used as a mobile power source in a broad spectrum of portable electric appliances. With recent great advances in the electronic, communication, and computer industries, high performance electronic and communication products, such as mobile phones, camcorders, laptop PCs, etc., have been developed, partly on the basis of lithium ion secondary batteries being available as power sources therefor. In advanced countries, such as America, Japan, and European countries, furthermore, active research has been conducted into power sources for hybrid automobiles, in which internal combustion engines are associated with lithium ion secondary batteries.
Readily commercially available, small lithium ion secondary batteries employ LiCoO2 for a cathode and carbon for an anode. LiCoO2 shows stable charge and discharge characteristics, excellent electron conductivity, high stability and flat discharge voltage characteristics, but the component cobalt is expensive due to small deposits, and is hazardous to the body. Thus, alternative materials are now required for the cathode of lithium ion secondary batteries. LiNiO2, LiCoxNi1-xO2, and LiMn2O4 are now under extensive study as cathode materials of lithium ion secondary batteries. LiNiO2 cannot be commercialized at present due to its difficulty in being stoichiometrically synthesized as well as its low thermal stability, although it has the same layered structure as that of LiCoO2. LiMn2O4 incurs a low cost in the production thereof and is environmentally friendly, but shows poor lifetime characteristics due to the structural phase transition Jahn-Teller distortion, and Mn dissolution, both attributable to Mn3+. Particularly, the Mn dissolution, resulting from the reaction of Mn with electrolytes, causes a great decrease in lifetime at high temperatures, acting as a hindrance to the commercialization of the rechargeable lithium ion battery.
Japanese Pat. Laid-Open Publication No. 2004-227790 discloses a lithium transition metal complex oxide having a spinel structure, produced as a cathode active material for secondary lithium ion batteries through sodium hydrogen carbonate coprecipitation, which shows a heat generation starting temperature of 220° C. or higher and excellent cell characteristics even under poor circumstances. Also, Japanese Pat Laid-Open Publication No. 2004-241242 discloses a lithium transition metal complex oxide having a spinel structure, produced as a cathode active material for secondary lithium ion batteries through sodium hydrogen coprecipitation, comprising a first and a second particle component, which are 1 to 50 μm and 8 to 50 μm, respectively. Japanese Pat. Laid-Open Publication No. discloses a spinel-type lithium manganese complex oxide having 5V capacity, represented by LiNi0.5Mn1.5O4, for cathode material, which is prepared using ammonium carbonate and shows high energy density and a superior life cycle even at high temperatures.
Conventional manganese carbonates, as described above, can be prepared using carbonate coprecipitation. However, crystals of the manganese carbonates thus obtained are not spherical, but are irregular in shape, with a broad particle distribution. Such irregularly shaped manganese carbonates with a broad particle distribution are poor in packing density and have a large specific surface in contact with the electrolytes, so that they dissolve in the electrolytes. Therefore, a higher packing density and a lower specific surface can be achieved with spheric monodispersed manganese carbonates. Spinel-type LiMn2O4 active materials, which are constant in shape, can be prepared with the spheric manganese carbonate serving as a starting material.